The previous post in this blog [LINK] went over my rough intuition of climate, primarily in middle latitudes like those of the US. Most of the broad categories that I described were largely aligned with the Köppen climate classification system (henceforth called the Köppen categorization). However, there is a more recent categorization known as the Trewartha climate classification system (henceforth called the Trewartha categorization) that is supposed to be more representative of middle latitudes like those of the US. Essentially, tropical, desert, and semi-arid climates, as well as polar and ice cap climates, are defined in the same ways between the two categorizations. The differences lie in the definitions of subtropical, continental, and subpolar oceanic/subarctic climates. One benefit of the Trewartha categorization is that it clearly separates boreal/subpolar climates from other oceanic and continental climates, whereas the Köppen categorization uses subcategories that could be a little more confusing. However, the definitions of subtropical, oceanic, and continental climates in the Trewartha categorization seem less justifiable to me. Follow the jump to see more details. Again, I am not a trained climatologist or meteorologist; I can't guarantee that this information is accurate, and I can only say that my intuitions seem through my limited understanding to align with superficial aspects of more detailed explanations.
In the Köppen categorization, a subtropical climate, labeled in the major group C, is defined as having a daily mean temperature in the coldest month that is at least 0 degrees Celsius. The third letter in the climate label depends on the daily mean temperature in the hottest month of the year, while the second letter depends on the amount & distribution of rainfall over the course of the year. Thus, a humid subtropical climate, labeled Cfa, must, in addition to satisfying the conditions of the major group C, also have no significant difference in rainfall over the course of the year, have at least 4 months having a daily mean temperature of at least 10 degrees Celsius, and have its hottest month have a daily mean temperature of at least 22 degrees Celsius. Also, a temperate oceanic climate, labeled Cfb, must, in addition to satisfying the conditions of the major group C, also have no significant difference in rainfall over the course of the year, have at least 4 months having a daily mean temperature of at least 10 degrees Celsius, and have its hottest month have a daily mean temperature below 22 degrees Celsius. Meanwhile, a Mediterranean-type climate, whose second letter is "s", has the driest month in the summer (in the second or third quarters in the calendar year in the northern hemisphere, with the opposite in the southern hemisphere), the wettest month in the winter, the driest month having at most 40 millimeters of precipitation, and the wettest month having at least 3 times as much precipitation as the driest month. Thus, a hot-summer Mediterranean-type climate, labeled Csa, must, in addition to satisfying the conditions of the major group C with the second letter "s", also have at least 4 months having a daily mean temperature of at least 10 degrees Celsius and have the hottest month have a daily mean temperature of at least 22 degrees Celsius. Also, a mild-summer Mediterranean-type climate, labeled Csb, must, in addition to satisfying the conditions of the major group C with the second letter "s", also have at least 4 months having a daily mean temperature of at least 10 degrees Celsius and have the hottest month have a daily mean temperature below 22 degrees Celsius.
In the Trewartha categorization, a subtropical climate, also labeled in the major group C, is defined as at least 8 months having a daily mean temperature of at least 10 degrees Celsius; as I understand, the implication is that any climate that fits into this category will not also have the daily mean temperature in the coldest month fall below 0 degrees Celsius. Thus, a humid subtropical climate, labeled Cf, must, in addition to satisfying the conditions of the major group C, also have no significant difference in rainfall over the course of the year. A Mediterranean-type climate, labeled Cs, in addition to satisfying the conditions of the major group C, follows the same rainfall pattern as in the Köppen categorization (for which I gave the labels Csa and Csb as examples in the Köppen categorization). By contrast, an oceanic climate, labeled Do, is defined as 4-7 months having a daily mean temperature of at least 10 degrees Celsius and all months having a daily mean temperature of at least 0 degrees Celsius; there are no distinctions based on precipitation patterns.
The fact that the Trewartha categorization, unlike the Köppen categorization, defines a humid subtropical climate based on having a large number of months with a moderately high daily mean temperature but without regard to the daily mean temperature in the hottest month seems a bit odd to me. In particular, if the Trewartha categorization were truly useful for this distinction, then there should be a large and diverse set of locations across the world in middle latitudes where at least 8 months have a daily mean temperature of at least 10 degrees Celsius but the daily mean temperature in the hottest month is below 22 degrees Celsius. The only cities I could find satisfying these conditions are the mild-summer Mediterranean climates (Csb in the Köppen categorization) or temperate oceanic climates (Cfb in the Köppen categorization) that exist in very narrow strips of land directly on the west coasts of continents in middle latitudes as well as the south coasts of some countries in the southern hemisphere. By country, these are cities between Eureka and Santa Barbara along the coast of California in the US, cities between Valparaíso and Concepción along the coast of Chile, Porto and cities to the north along the coast of Portugal, cities on the west and north coasts of Spain, cities between Biarritz and La Rochelle along the coast of the Bay of Biscay in France, cities between Cape Town and Gqeberha along the coast of the Indian Ocean in South Africa, Melbourne in Australia, and cities on the North Island (including Auckland and Wellington) of New Zealand. In the Trewartha categorization, the label Cf would be used for the Köppen categorization Cfb while the label Cs would be used for the Köppen categorization Csb for this restricted set of locales. Thus, I don't see the benefit of the Trewartha categorization in terms of insisting that a subtropical climate must have at least 8 months with a daily mean temperature of at least 10 degrees Celsius without regard for the daily mean temperature in the hottest month.
More broadly, the Trewartha categorization would lump together, under humid subtropical climates labeled Cf, cities that have very different latitudes, climates, and positions relative to their continents & coasts, in ways that seem nonsensical to me. Specifically, the Trewartha categorization would lump together cities like La Rochelle & Biarritz in France together with cities like Houston or Atlanta in the US. This puzzles me because La Rochelle & Biarritz are on the west coast of France, so they consistently get mild moisture from the prevailing westerlies coming over the Atlantic Ocean; as a result, rain tends to come consistently over many days as a steady drizzle while the typical high & low temperatures are kept moderated. By contrast, Houston & Atlanta are more inland in the US, far from the west coast, and most of their moisture comes from sporadic winds from the Gulf of Mexico; as a result, rain doesn't come consistently over many days, but when it comes, it is more intense, while the typical highs are much higher & lows are a bit lower due to the less consistent presence of moisture to moderate these temperatures. Essentially, I see the focus of the Köppen categorization on the daily mean temperature in the hottest month as being more representative of the typical climate, because it shows how extreme the temperatures can be in the absence of steady moisture; by only focusing on overall moisture levels and the number of months with a moderately high daily mean temperature without considering the hottest month, the Trewartha categorization loses this granularity, so I believe the Trewartha categorization doesn't really fulfill its stated mission of more accurately describing climates in middle latitudes.
Similarly, the Trewartha categorization would lump together, under oceanic climates labeled Do, cities that have very different latitudes, climates, and positions relative to their continents & coasts, in ways that seem nonsensical to me, especially because the label Do doesn't account for differences in precipitation patterns over the course of the year. I raise this issue especially knowing that an older version of the Wikipedia article about the Trewartha categorization claimed that it was specifically created to remove the deficiency that cities like Seattle & Santa Barbara would have the same label in the Köppen categorization (Csb) despite Seattle getting much more rain, having much cooler winters, and having much different vegetation than Santa Barbara; under the Trewartha categorization, Santa Barbara would have the label Cs (Mediterranean-type climate), while Seattle would have the label Do (oceanic climate). With this in mind, I find that there are collections of cities with the same label in the Trewartha categorization that are much more problematic than Seattle & Santa Barbara having the same label in the Köppen categorization for the following reasons. Specifically, in the continental US, the Trewartha categorization would lump together cities like Seattle & Yreka together with cities like New York & DC in the US under the label Do (oceanic climate) just because all of these cities have all months with a daily mean temperature of at least 0 degrees Celsius and 4-7 months with a daily mean temperature of at least 10 degrees Celsius. This puzzles me because Seattle & Yreka are on the west coast, so they consistently get mild moisture from the prevailing westerlies coming over the Atlantic Ocean; as a result, rain tends to come consistently over many days as a steady drizzle while the typical high & low temperatures are kept moderated. By contrast, New York City & DC are on the east coast, far from the west coast, and most of their moisture comes from sporadic winds from the south/southwest over the Atlantic Ocean; as a result, rain doesn't come consistently over many days, but when it comes, it is more intense, while the typical highs are much higher & lows are much lower due to the less consistent presence of moisture to moderate these temperatures. Once again, I see the focus of the Köppen categorization on the daily mean temperature in the hottest month as being more representative of the typical climate, because it shows how extreme the temperatures can be in the absence of steady moisture; by only focusing on overall moisture levels and the number of months with a moderately high daily mean temperature without considering the hottest month, the Trewartha categorization loses this granularity, so I again believe the Trewartha categorization doesn't really fulfill its stated mission of more accurately describing climates in middle latitudes. Moreover, the fact that Seattle & Yreka have the same label under the Trewartha categorization could be argued as just as much a problem as Seattle & Santa Barbara having the same label under the Köppen categorization because Yreka gets half as much rain per year as Seattle and about as much rain as Sacramento, the latter of which is close to the boundary with places that have semi-arid climates (under both categorizations) like Stockton & Fresno; Yreka isn't considered semi-arid only because of the cooler temperatures overall (as cooler air has a lower equilibrium partial pressure of water vapor, so the threshold of precipitation below which a climate is considered semi-arid drops with temperature). Additionally, while Seattle technically gets as much rain over the course of the year as New York City or DC, I would think that the rain in Seattle being concentrated in the winter and being much more steady each day during that season should lead to much different temperature dynamics & types of vegetation than in New York City or DC where there is consistent precipitation through the year, more in the summer than in the winter, but the precipitation is less consistent hour-by-hour or day-by-day when it does come & more intense when it does come. Finally, the fact that Seattle has such dry summers makes it much more vulnerable to extreme high temperatures & wildfires, similar to Yreka or even parts of southern California, than New York City or DC, so I think the fact that Seattle has the same label as Yreka or even Santa Barbara under the Köppen categorization is actually more justifiable given the much more significant effects of dry summer conditions & wildfires on vegetation & human habitation.
(UPDATE: I've also seen that the Trewartha categorization has optional letters to denote the daily mean temperature in the warmest and coldest months, which goes even beyond the Köppen categorization. I'm not sure whether this is consistently used, because on Wikipedia, any mention of the Köppen categorization uses the full 3-letter label (except for tropical or polar climates which are defined with only 2 letters), while almost all mentions of the Trewartha categorization only use 2-letter labels without using the optional additional letters; that said, the few climatology papers that I've seen seem to only use 2-letter labels in the Köppen categorization too. Therefore, if I'm going to use the full 3-letter labels in the Köppen categorization, then it is better to be fair & consistent by using full 4-letter labels in the Trewartha categorization.
Specifically, in the previous examples, the labels under the Köppen categorization for La Rochelle and Biarritz in France are Cfb and for Houston and Atlanta in the US are Cfa; the corresponding labels under the Trewartha categorization are respectively Cfbk, Cfbk, Cfhl, and Cfak. This addresses the concern that the Trewartha categorization doesn't otherwise distinguish climates in subgroup Cf by how hot the summer is, which can give information about whether a location at a middle latitude is on the west coast, which would imply consistent light rain & consequently moderated hot & cold temperatures, or somewhere else, which would imply more sporadic but intense rain & consequently more extreme hot & cold temperatures. Similarly, in the US, the labels under the Köppen categorization for Seattle and Yreka are Csb and for New York City and DC are Cfa; the corresponding labels under the Trewartha categorization are respectively Dobk, Dobk, Doak, and Doak. This again addresses the aforementioned concern about summer temperature distinctions and moisture levels, suggesting that consistent use of 4-letter labels under the Trewartha categorization could alleviate problems with moving away from 3-letter labels under the Köppen categorization. However, it doesn't address the concern that Seattle and Yreka still have the same 4-letter label under the Trewartha categorization despite Yreka getting half as much rain per year as Seattle and being much closer in rainfall totals than Seattle to many semi-arid climates only marginally to the south in California, the concern that Seattle is considered just as "oceanic" in the Trewartha categorization as New York City or DC despite having a distinct dry season in the summer that affects vegetation & water availability, or the concern that Seattle's vulnerability in its dry summers to heat waves & wildfires arguably makes its climate more similar to that of Santa Barbara (as reflected in the Köppen categorization as they share the label Csb, but not reflected in the Trewartha categorization, as Seattle and Santa Barbara have the respective 2/4-letter labels Do/Doak and Cs/Csbl). Especially as Seattle has over the last several years experienced more droughts, intense heat waves of temperatures going much higher than the extremes found on the east coast of the US, and wildfires much like California, it would be better for those climates to be categorized similarly, and I would think that categorizing the climate of the Pacific Northwest region of the US specifically around the west coast, the Willamette Valley, and the areas around Seattle, to be more similar to areas in the east coast of the US would be misleading particularly in the context of mitigating the effects of climate change.)